bluetooth: controller: abstract PPI handling for LE Coded

This commit completes the refactoring of radio.c, so it
calls abstract functions for PPI handling for LE Coded
PHY (e.g. enable, disable channels, or configure endpoints),
which, then, call the platform-specific functions for PPI
handling. The abstract functions are simple wrappers,
implemented in radio_nrf5_ppi.h, conditionally, if we
build the controller with support for LE Coded PHY.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>